The viscosity-temperature relationship of a lubricating oil is one of the critical criteria which must be considered when selecting a lubricant for a particular application. Viscosity Index (VI) is an empirical, unitless number which indicates the rate of change in the viscosity of an oil within a given temperature range. Fluids exhibiting a relatively large change in viscosity with temperature are said to have a low viscosity index. A low VI oil, for example, will thin out at elevated temperatures faster than a high VI oil. Usually, the high VI oil is more desirable because it has higher viscosity at higher temperature, which translates into better or thicker lubrication film and better protection of the contacting machine elements. In another aspect, as the oil operating temperature decreases, the viscosity of a high VI oil will not increase as much as the viscosity of a low VI oil. This is advantageous because the excessive high viscosity of the low VI oil will decrease the efficiency of the operating machine Thus high VI (HVI) oil has performance advantages in both high and low temperature operation. VI is determined according to ASTM method D 2270-93 [1998]. VI is related to kinematic viscosities measured at 40° C. and 100° C. using ASTM Method D 445-01.
In order to provide step-out fuel economy while maintaining or improving other lubricant performance features, basestocks with lower friction coefficients are needed. Low friction coefficients and low viscosities at all temperature ranges are two key properties contributing to lubricant fuel economy.
Synthetic lubricants having found increasing use as high performance basestocks. Because they are synthesized from relatively pure hydrocarbon components, synthetic lubricants tend to be more consistent in their compositions as compared to lubricant basestocks derived from crude oil refining processes. As such, their properties can be more readily controlled.
For example, polyalphaolefins (PAO) comprise a class of synthetic hydrocarbon lubricants, manufactured by the catalytic oligomerization (polymerization to low molecular weight products) of linear alpha-olefins (LAOS) typically ranging from 1-hexene to 1-octadecene, more typically from 1-octene to 1-dodecene, with 1-decene as the most common and often preferred material. Such fluids are described, for example, in U.S. Pat. No. 6,824,671 and patents referenced therein.
However, because PAOs are oligomerization/polymerization products, they exhibit distributions of molecular weights, which can negatively affect some aspects of their performance as lubricants. Likewise, in order to render PAOs suitable for use as lubricants, it is necessary to reduce the unsaturation of the as-polymerized carbon chains of the PAO products. Accordingly, it is conventional to hydrogenate these as-polymerized PAO products in order to reduce the level of unsaturation in the molecules, so as to render them suitable for use as lubricant basestocks, which is a costly process.
Another class of synthetic lubricant fluids useful as lubricant basestocks are polyalkylene glycols (PAG), which can often demonstrate improved friction coefficients, even as compared to PAO fluids. Some of these materials are described in the references set forth below.
Japanese Patent Application No. 51-070396 discloses lubricants containing a polyalkylene glycol ester with cyclohexanedicarboxylic acid which are heat resistant and useful for finishing undrawn nylon 6 or polyester fibers. The undrawn fibers were coated with an emulsion containing 10% of a composition of polyethylene glycol monolauryl ether 1,2-cyclohexanedicarboxylate, polyethylene glycol stearyl ether, polyethylene glycol castor oil ester and isohexadecyl phosphate Na salt, stored 24 hours at 20° and 65% relative humidity. No fume generation occurred on passing the resulting fibers over a plate at 180° and 100 m/min, whereas fume generation occurred on finishing the fibers with a comparative lubricant.
Japanese Patent Application No. 52-121596 discloses heat resistant lubricants for finishing of polyester fibers, which were prepared by blending a cyclohexanedicarboxylate or cyclohexenedicarboxylate of an alcohol containing alicyclic alkyl groups, optionally polyoxyalkylated, with a lubricant composition. The undrawn polyester fibers were coated with a 10% emulsion of a lubricant containing 70% diester of cyclohexanol-propylene oxide adduct with 1,2-chclohexanedicarboxylic acid to give drawn fibers without fume generation.
U.S. Pat. No. 5,370,809 discloses a synthetic lubricating oil comprising as a base oil at least one kind of carbonic acid ester represented by general formula (I):(X1—O)b—B—[O-(A1O)c—C(O)O-(A2O)d—R1]a,wherein X1 is a member selected from the group consisting of a hydrogen atom, an alkyl group, a cycloalkyl group or a group having the formula R2—OA3)e-, A1, A2 and A3 are identical or different from each other and are each an alkylene group having 2-4 carbon atoms, R1 and R2 are identical or different from each other and are each a hydrogen atom, an alkyl group or a cycloalkyl group, B represents the residue of a compound having 3-20 hydroxyl groups, a is an integer of 1-20, b is an integer of 0-19, and the sum of a and b equals an integer of 3-20, c and d are each an integer of 0-50 with the proviso that c and d are not both zero and e is an integer of 1-50.
Japanese Patent Application No. 60-181142 discloses modified diesters of the formula R(ZO)mO2Z1CO2(ZO)nR, wherein R=C3-15 alkyl; Z=C2-4 alkylene; Z1=C2-8 aliphatic or alicyclic dibasic acid residue; m, n=1,7; the sum of the number of C atoms in Z and R is 5-17, which show excellent plasticizing efficiency, migration and volatilization resistance, static charge dissipation and fogging resistance in chlorine-containing vinyl polymer compositions.
U.S. Pat. No. 6,239,298 discloses a fuel lubricity additive, made by a two-step process wherein the first step is co-reacting an unsaturated base oil, predominantly from vegetable oil sources, and a compound having a diene structure and a carboxylic acid group, wherein the second step is esterifying or amidifying the free carboxylic acid group of anhydride group with a poly-hydroxy-containing compound or polyamine compound to form the final fuel lubricity additive useful in diesel fuels. The inventive fuel lubricity additive also is useful as a dispersant.
U.S. Published Patent Application No. 2006/0183832 discloses a plasticizer for amorphous polyester resin which can give softness without hindering the transparency of the amorphous polyester resin, and an amorphous polyester resin composition superior in softness, transparency and heat resistance. A plasticizer for amorphous polyester resin, which is made of an ester of an (Aa) component selected from hydroxy aromatic carboxylic acid (AI), hydroxy condensed polycyclic aromatic carboxylic acid, hydroxy alicyclic carboxylic acid and others, and an (Ab) component selected from aliphatic alcohol, alicyclic alcohol, aromatic alcohol, phenol, alkylphenol, or alkylene oxide added products thereof.
However, PAG fluids, like PAOs, are also oligomer/polymer products of epoxides which have undesirable molecular weight distributions. Ethylene oxide or propylene oxide based PAG products are water soluble and hydrocarbon incompatible. Some of the detrimental aspects of PAG molecules includes oil miscibility (compatibility), elastomer compatibility and hygroscopicity.
There is a desire to have single molecule with precise structure for low viscosity basestocks. Thus, despite recent advances, there remains an unmet need in the art to develop synthetic materials suitable for use as a lubricant basestocks, which combine the advantages of both PAO and polyalkylene glycol fluids, but are discrete compounds having specific molecular weights.